The present invention relates to systems for controlling the usage of electrical energy, and in particular to a method and system for controlling the usage of electrical energy on the basis of demand and/or on a timed basis.
It is not uncommon for industrial and large users of electrical energy to be charged not only on the basis of power consumption but also on the basis of peak demand placed on the electrical supply system. For example, electrical demand may be established in a short measurement period of 15,30 or 60 minutes to establish a demand rate for the electrical billing period. This demand rate is then factored into the users bill for electrical power during that electrical billing period.
Accordingly, control of electrical demand is becoming more widespread and is encouraged by electrical energy suppliers since the limiting of peaks of demand limits the need for additional equipment in the energy supply network. Such control usually takes the form of monitoring demand and selectively disconnecting loads from the electrical supply system if demand equals or exceeds some targeted value.
The typical demand control system makes use of a switching device such as a relay between the load and the electrical supply source. The switch is normally closed so that the load is normally connected to the power source and can receive power therefrom on demand. If a targeted demand value is equalled or exceeded, the switch is then opened to disconnect the load and thus reduce demand. A plurality of loads is ordinarily connected to a source in this fashion and the loads are arranged in a priority ordering scheme so that the loads are disconnected in sequence from lowest to highest priority until the demand target value is again achieved.
The demand controller is typically arranged as indicated with a normally closed switching arrangement so that failure of the controller caused by, for example, loss of power or the like, will place all the loads in connection with the power source. It will be appreciated, however, that such an arrangement results in an uncontrolled demand for the period of the failure. If the failure continues for 30 minutes or a period exceeding the measurement period of demand, the target rate for demand may be exceeded for the entire measurement period and a high demand rate will be established for that electrical billing period. This high demand rate may be troublesome for periods of up to one year or longer depending upon how the supplier involved factors in demand. Thus, merely because of a failure of short duration, a high demand rate for billing purposes may be established and may result in extremely high utility bills for a long period of time.
It is accordingly an object of the present invention to provide a novel method and system for controlling demand of a plurality of loads connected to a source of electrical power without the foregoing disadvantages of the prior art in the event of temporary system failures.
It is yet another object of the present invention to provide a novel method and system for controlling demands placed on an electrical supply system by a plurality of loads wherein, in the event of an isolated failure affecting the controller, provision is made for the automatic emergency control of demand levels and an alarm or notification that a failure has occurred.
In demand control systems such as discussed above, it is often desirable to incorporate control of demand on a timed, periodic basis in addition to the normal control on a demand basis. Thus, for example, the controller might ordinarily operate on a demand basis during the main daytime hours and then may revert to a purely time controlled basis during some predetermined portion of the nighttime hours.
It is accordingly a further object of the present invention to provide a novel method and system for controlling the connection of a load to a power source as a function of both demand and time.
It is yet another object of the present invention to provide a novel method and system for controlling the disconnection of a load from a power source as a function of demand and time or merely just demand wherein these alternative modes of control are selectable for each load.
These and other objects and advantages are accomplished according to the present invention as will become apparent to one skilled in the art to which the inventors pertains from the following detailed description when read in conjunction with the appended drawings.